Non-reciprocal circuit devices such as isolators are used in mobile communications equipments utilizing frequencies from several hundreds MHz to ten-odds GHz, such as base stations and terminals of cell phones, etc. Isolators disposed between power amplifiers and antennas in mobile communications equipments are required to have excellent insertion loss characteristics, reflection loss characteristics and isolation characteristics to prevent unnecessary signals from returning to power amplifiers at the time of transmission, and to stabilize the impedance of power amplifiers on the load side.
As such a non-reciprocal circuit device, an isolator shown in FIG. 18 is conventionally well known. This isolator comprises three central conductors 21, 22, 23 crossing at an angle of 120° with electric insulation on one main surface of a ferrimagnetic microwave ferrite 30. Each central conductor 21, 22, 23 has one end connected to the ground and the other end connected to a matching capacitor C1-C3. A terminal resistor Rt is connected to a port (for instance, P3) of one of the central conductors 21, 22, 23. A DC magnetic field Hdc is applied from a permanent magnet (not shown) to the ferrite 30 axially. This isolator functions such that high-frequency signals input from a port P1 are transmitted to a port P2, while reflected waves entering a port 2 are prevented from being transmitted to a port P1 by absorption by the terminal resistor Rt, so that unnecessary reflected waves generated by the impedance variation of an antenna are prevented from entering a power amplifier, etc.
Recently much attention has been paid to an isolator constituted by a different equivalent circuit from those of conventional three-terminal isolators, which has excellent insertion loss characteristics and reflection characteristics. For instance, the isolator described in JP 2004-88743 A is called “two-terminal isolator,” which has two central conductors. FIG. 19 shows the equivalent circuit of its basic structure. This two-terminal isolator comprises a first central conductor (first inductance element) L1 electrically connected between a first input/output port P1 and a second input/output port P2; a second central conductor (second inductance element) L2 crossing the first central conductor L1 with electric insulation and electrically connected between the second input/output port P2 and the ground; a first capacitance element C1 electrically connected between the first input/output port P1 and the second input/output port P2 to constitute a first parallel resonance circuit with the first central conductor L1; a resistance element R; and a second capacitance element C2 electrically connected between the second input/output port P2 and the ground to constitute a second parallel resonance circuit with the second central conductor L2.
The first parallel resonance circuit sets a frequency at which the isolation (reverse attenuation characteristics) is the maximum, and the second parallel resonance circuit sets a frequency at which the insertion loss is the minimum. When high-frequency signals are transmitted from the first input/output port P1 to the second input/output port P2, resonance does not occur in the first parallel resonance circuit between the first input/output port P1 and the second input/output port P2, but the second parallel resonance circuit is resonated, resulting in small transmission loss (excellent insertion loss characteristics). Current reversely flowing from the second input/output port P2 to the first input/output port P1 is absorbed by the resistance element R connected between the first input/output port P1 and the second input/output port P2.
FIG. 20 shows a specific example of the structure of a two-terminal isolator. This two-terminal isolator 1 comprises metal cases (upper case 4 and lower case 8) made of a ferromagnetic material such as soft iron to constitute a magnetic circuit, a permanent magnet 9, a central conductor assembly 30 comprising a microwave ferrite 20 and central conductors 21, 22, and a laminate substrate 50 on which the central conductor assembly 30 is mounted.
The upper yoke 4 for containing the permanent magnet 9 is substantially in a box shape having an upper surface 4a and four side surfaces 4b. The lower yoke 8 has a bottom surface 8a and a pair of side surfaces 8b. Each surface of the upper and lower yokes 4, 8 is properly plated with a conductive metal such as Ag, Cu, etc.
The central conductor assembly 30 comprises a disc-shaped microwave ferrite 20, and first and second central conductors 21, 22 perpendicularly crossing on an upper surface of the microwave ferrite 20 via an insulating layer (not shown), such that the first and second central conductors 21, 22 are electromagnetically coupled to each other at an intersection. Each of the first and second central conductors 21, 22 is constituted by two lines, and both end portions thereof are separated from each other and extend under the microwave ferrite 20.
FIG. 21 is an exploded view of the laminate substrate 50. The laminate substrate 50 is constituted by a dielectric sheet 41 having connecting electrodes 51-54 connected to ends of the central conductors 21 and provided with capacitor electrodes 55, 56 and a resistor 27 on the rear surface, a dielectric sheet 42 provided with a capacitor electrode 57 on the rear surface, a dielectric sheet 43 provided with a ground electrode 58 on the rear surface, a dielectric sheet 45 provided with an external input electrode 14, an external output electrode 14 and external ground electrodes 16, etc.
The central-conductor-connecting electrode 51 corresponds to the first input/output port P1 in the above equivalent circuit, and the central-conductor-connecting electrodes 53, 54 correspond to the second input/output port P2. One end of the first central conductor 21 is electrically connected to the external input electrode 14 via the first input/output port P1 (central-conductor-connecting electrode 51). The other end of the first central conductor 21 is electrically connected to the external output electrode 14 via the second input/output port P2 (central-conductor-connecting electrode 54). One end of the second central conductor 22 is electrically connected to the external output electrode 14 via the second input/output port P2 (central-conductor-connecting electrode 53). The other end of the second central conductor 22 is electrically connected to the external ground electrode 16. The first capacitance element C1 is electrically connected between the first input/output port P1 and the second input/output port P2 to constitute a first parallel resonance circuit with the first central conductor L1. The second capacitance element C2 is electrically connected between the second input/output port P2 and the ground to constitute a second parallel resonance circuit with the second central conductor L2.
To provide multi-functional, lightweight cell phones, the miniaturization of their parts is strongly demanded. As non-reciprocal circuit devices are demanded to be as small as about 2.5 mm×2.5 mm×1.0 mm, the microwave ferrite 20, for instance, is also demanded to be as small as having an overall size of about 1.0 mm×1.0 mm×0.15 mm. However, the miniaturization of the microwave ferrite 20 invites decrease in the inductance of inductors constituted by central conductors.
If the microwave ferrite 20 were made small like this, practically useful characteristics cannot be obtained in the three-terminal, non-reciprocal circuit device shown in FIG. 18. Although the two-terminal isolator described in JP 2004-88743 A, which is shown in FIG. 19, has better electric characteristics than those of the three-terminal non-reciprocal circuit device, its insertion loss exceeds 1 dB in the passband, unsatisfactory for practical applications.
To obtain a non-reciprocal circuit device having excellent electric characteristics, various factors generated in the products, such as parasitic inductance, floating capacitance, etc. should be taken into consideration. Even if the above two-terminal isolator were ideally designed, parasitic inductance, floating capacitance, etc. would occur in the first and second parallel resonance circuits for structural reasons in its operation, resulting in impedance deviated from the designed level. To avoid the deterioration of insertion loss characteristics and isolation characteristics due to impedance mismatching with other connected circuits, it is necessary to find optimum values through repeated test production, taking a long period of time for the development of products.
Because the first and second central conductors 21, 22 are coupled to each other, the inductance also changes therewith. Even if the width, intervals, etc. of lines constituting them were changed taking unnecessary reactance components into consideration, it is difficult to separately adjust the input impedance of the first and second input/output ports P1, P2, failing to find optimum conditions of matching with external circuits. Particularly the deviation of the input impedance of the first input/output port P1 is undesirable because it causes increase in the insertion loss.